The invention relates to lead-free optical glasses which have refractive indices nd of between 1.65 and 1.80 and Abbe numbers xcexdd of between 21 and 33. These glasses belong to the optical glass type consisting of the heavy flint glasses (HF).
Since, in recent years, the glass components PbO and As2O3 have been considered to be environmentally polluting in public discussions, the manufacturers of optical apparatuses also require PbO-free and preferably also As2O3-free glasses having the respective optical properties.
It is desirable to dispense with PbO also for the production of light glass parts, i.e. of glasses having a low density.
It is as a rule not possible to reproduce the desired optical and technical glass properties influenced by PbO by simply replacing lead oxide by one or more components. Instead, new developments or wide-ranging changes in the glass composition are necessary.
The patent literature already includes some publications in which lead-free glasses having optical values from said range are described. However, these glasses have a very wide range of disadvantages.
DE 32 164 51 A describes optical lightweight glasses having a refractive index nd of  greater than 1.70 and an Abbe number xcexdd of xe2x89xa722 and a density xcfx81 of xe2x89xa63.5 g/cm3. These glasses contain up to 3% by weight of B2O3, a component which is aggressive towards Pt. If such glasses are melted in Pt crucibles or if they come into contact with other Pt components, which would improve the homogeneity and low bubble count of the glasses, they have a higher level of Pt impurities, with the result that their transmittance is adversely affected.
U.S. Pat. No. 3,589,918 describes an optical glass for lens systems comprising the glass system SiO2xe2x80x94K2Oxe2x80x94TiO2xe2x80x94Sb2O3. The high Sb2O3 contents of up to 45% by weight in this glass make the glass susceptible to separation and heavy and adversely affect its transmittance so that it is not suitable for modern applications in optics.
JP 53-16718 A discloses glasses having high contents of divalent oxides (MO=MgO+CaO+SrO+BaO+ZnO 15-50% by weight) and relatively low contents of TiO2 (1-25% by weight). These glasses have Abbe numbers of between 30 and 45. Owing to the high MO content, their stability to crystallization is low.
JP 52-25812 A discloses TiO2- and Nb2O5-glasses whose compositions vary over a wide range. According to the examples, the glasses have very high (25-45% by weight) or very low (5% by weight) of Nb2O5 contents. The same applies to the MO content (21 and 30% by weight and 0-5% by weight, respectively). These glasses having TiO2 contents of up to 50% by weight are also not sufficiently stable to crystallization for economical continuous production.
It is an object of the invention to provide lead-free optical glasses having a refractive index nd of between 1.65 and 1.80 and an Abbe number xcexdd of between 21 and 33, which possess good melting and processing properties and have good chemical resistance, good stability to crystallization and a low density.
This object is achieved by glasses described in Patent claim 1.
The good fusibility meltability of the glasses is achieved by the balanced proportions of fluxes (Na2O, K2O) to glass formers (SiO2+optionally B2O3, Al2O3) in relation to the poorly melting highly refractive components (MO (BaO, CaO+optionally SrO, MgO), TiO2, Nb2O5, WO3+optionally ZrO2)
The glasses contain 27 to 40% by weight of the main glass former SiO2. In the case of higher proportions, the desired high refractive index would not be reached and the fusibility would deteriorate; in the case of lower proportions, the stability to crystallization and the chemical resistance would be reduced. An SiO2 content of at least 29% by weight is preferred, particularly preferably of at least 31% by weight of SiO2. A content of not more than 36% by weight is particularly preferred.
For further stabilization, the glasses may contain up to 6% by weight of Al2O3, preferably up to  less than 3% by weight of Al2O3, and up to  less than 0.5% by weight of B2O3. Higher proportions of glass formers would reduce the fusibility. Preferably, Al2O3 is dispensed with. It is a major advantage that the B2O3 content can remain limited to said low proportions, since the aggressiveness of the glass melt is thus reduced, so that glasses containing extremely small amounts of Pt impurities and hence having very high transmittances can be produced in Pt components.
In order to achieve the desired optical position of a heavy flint glass, relatively high proportions of highly refractive components are required. The proportion of the glass formers and of fluxes having a low refractive index (Na2O, K2O) is therefore limited. Preferably 69.5% by weight of SiO2+Al2O3+B2O3+Na2O +K2O are not exceeded, and very particularly preferably the limit of this sum is max. 60.5% by weight.
In addition to the glass formers, the glasses contain a proportion of fluxes which is sufficient for good fusibility. Thus, they contain at least 8% by weight and not more than 28% by weight of Na2O+K2O, and in particular 7-18% by weight of Na2O and 1-10% by weight of K2O. A flux content of 12-26% by weight with 9-16% by weight of Na2O and 3-10% by weight of K2O is preferred, and at least 14% by weight of Na2O+K2O with 10-15% by weight of Na2O and 4-9% by weight of K2O are particularly preferred. A sum of Na2O and K2O of not more than 21% by weight is very particularly preferred.
The glasses contain the following highly refractive components:
They contain 1.5- less than 15% by weight of alkaline earth metal oxides, preferably 3.5-14, especially xe2x89xa611, particularly preferably xe2x89xa610, % by weight.
Specifically:
1-10% by weight of BaO, preferably 3-10, particularly preferably 3-8, % by weight
0.5-5% by weight of CaO, preferably 0.5-3% by weight
0-3% by weight of MgO, preferably 0- less than 2% by weight, preferably Mgo-free
0-3% by weight of Sro, preferably 0- less than 2% by weight, preferably SrO-free
The proportion of alkaline earth metal oxide is limited to said maximum content since a further increase would be possible only by reducing the glass former and flux content and would lead to crystallization effects, particularly since the further components which increase the refractive index are comparatively good nucleating agents. Said minimum contents of the alkaline earth metal oxides are necessary in order to establish the high refractive index and to stabilize the chemical resistance.
The glasses contain 21-37% by weight of TiO2, preferably 23-35, particularly preferably 26-33, % by weight.
The glasses furthermore contain 5-17% by weight of Nb2O5, preferably  greater than 5, especially 7-15, particularly preferably xe2x89xa612, % by weight.
These two components form the basis of the high refractive index at the desired Abbe number. An increase in the TiO2 content would reduce the Abbe number excessively and also excessively increase the tendency to crystallization. An increase in the Nb2O5 content would increase the Abbe number to a very excessive extent and slightly reduce the refractive index.
For stabilization to crystallization, the glasses may contain up to 7% by weight of ZrO2, preferably  less than 5% by weight. Preferably, the ZrO2 content replaces a corresponding part of the TiO2 content, so that preferably the maximum sum of TiO2+ZrO2 is 37% by weight, in particular 35% by weight.
A further increase of ZrO2 would in turn lead to an increase in the tendency to crystallization; furthermore the optical position would be undesirably shifted.
By parallel use of different nucleating agents and crystal formers, namely TiO2 in addition to Nb2O5 and optionally ZrO2, the formation of defined crystals is impeded and it is possible to achieve the desired exceptional refractive index by means of high proportions of these components without adding lead.
An important component is WO3. It is present in an amount of 0.1 to 7% by weight in the glass and, in addition to the fine adjustment of the optical position, serves for further reducing the tendency to crystallization by its spatial coordination, which is unusual in this glass system. This too impedes the formation of defined crystals. In this glass system, a higher WO3 content would in turn undesirably shift the optical position. A WO3 content of between 0.2 and 5% by weight is preferred, particularly preferably between 0.2 and 4% by weight.
In order to improve the glass quality, one or more refining agents known per se can be added in the customary amount to the mixture for refining the glass. Thus, the glass has a particularly good internal glass quality with respect to freedom from bubbles and freedom from stria.
If, instead of As2O3, for example, Sb2O3 is used as refining agent, preferably an amount of up to 1% by weight, which is possible without losses in respect of the glass quality, the glasses which are lead-free according to the invention are additionally arsenic-free.
The glasses may also contain, for example, up to 1% by weight of Fxe2x88x92 and/or up to 1% by weight of Clxe2x88x92. Fxe2x88x92 is added, for example, as KF or KHF2. Clxe2x88x92 is added, for example, as NaCl.
The glasses from said composition range have refractive indices nd of between 1.65 and 1.80 and Abbe numbers xcexdd of between 21 and 33. Glasses from the composition ranges stated in each case as being preferred have refractive indices of nd of between 1.68 and 1.79 and Abbe numbers xcexdd of between 23 and 32. The refractive indices nd and the Abbe numbers xcexdd of glasses from the ranges stated as being particularly preferred are between 1.70 and 1.79 and between 24 and 28.
The entire disclosure of all applications, patents and publications, cited herein and of corresponding German application No. 10133763.9, filed Jul. 11, 2001 are incorporated by reference herein.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
In the foregoing and in the following examples, all temperatures are set forth uncorrected in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.